Observe and analyze the properties of polarized light.
If you wear polarized sunglasses, and look at the reflection of sunlight off of water, the glare from the reflected light will almost disappear. Since light is a transverse wave it can be vibrating vertically, horizontally or any angle in between.
The transverse waves in sunlight are oriented randomly. When sunlight reflects off of certain surfaces, the light oriented in one direction is not reflected as well as light reflected 90 degrees from that angle. This makes it partially polarized.
Polarized filters are designed to let polarized light pass through at one angle, but block light from passing through 90 degrees from that angle.